1. Field of the Invention
This invention pertains generally to the field of test procedures and apparatus used to determine whether hearing impairment exists in an individual without the voluntary response of the subject.
2. Description of the Prior Art
Detection of hearing impairment in an individual usually takes place as the individual matures and it is observed that he or she is not responding normally to aural stimuli. A child may be several years old before his hearing impairment is fully recognized, and his educational development will have been consequently delayed and possibly permanently impaired. It is therefore highly desirable to screen children at as young an age as possible, preferably soon after birth and before they are initially released from the hospital. However, it is obvious that babies cannot voluntarily communiate with the person who examines them and thus some other means must be found to determine whether the baby is hearing an auditory signal.
It has been found that the presence of certain audible signals will produce changes in the electroencephalographic (EEG) signals of a person whose brain is in fact responding to a sound detected by the person's ears. Unfortunately, the response signals present in an electroencephalogram are relatively small in magnitude compared to general EEG brain activity levels. Techniques such as signal averaging have been used for diagnostic evaluations to separate the response signal from the background noise, but the equipment required is relatively expensive and a fairly high level of personnel training is required for reliable identification and interpretation of the average response waveform. While the time and cost involved for such tests can be justified for diagnostic purposes, they are not practical for mass screenings.